Sum all of the items in a JSONB array field

Question

If my table is setup like:

indicators:
  id: 56789,
  funding (JSONB): [
    {
      amount: 345678
    },
    {
      amount: 7899
    }
  ]

I can successfully sum the first amounts on every record with:

Rails — Indicator.sum("(funding->0->>'amount')::float")

SQL — SELECT SUM((funding->0->>'amount')::float) FROM "indicators"

How would you query a sum of all of the amounts (not just the 0 index items)?

Running Rails 5 & Postgres 9.5.4.

Note: This post is similar to How do I query using fields inside the new PostgreSQL JSON datatype? - but I'm looking for a way to iterate over each of the array elements to sum them (as opposed to calling them directly by an index number).

Update... Thanks to @klin's answer below, I was able to put together the Rails version that gave me the aggregate total:

Indicator.joins("cross join lateral jsonb_array_elements(funding)").sum("(value->>'amount')::float")

Answer 1

Use jsonb_array_elements()

select sum((value->>'amount')::float)
from indicators
cross join lateral jsonb_array_elements(funding)